For the IC manufacturing process, the highly précised projection objective is used to project the pattern on the mask to the wafer with resist. The existing deep ultraviolet (DUV) lithography takes the laser source of 193 nm wavelength. With the assistance of resolution enhancement technology like off-axial illumination, phase shift mask, and optical proximity correction, the DUV lithography can achieve the 45 nm node of the industrial IC manufacturing. But for the 32 nm node and below, extreme ultraviolet (EUV) lithography is the most promising next generation tech. The wavelength of EUV lithography is about 11˜15 nm. Like the DUV lithography, deep ultraviolet lithography (DUVL) also takes the step-scan model.
EUVL system includes plasma source, reflective illumination system, reflective mask, reflective projection objective, and the wafer covered with EUV resist, and the scanner. The ray beams emerged from the source get reshaped and uniformed by the illumination system, and then illuminate on the reflective mask. The reflected ray beams from the mask incident into the projection objective, and finally generate a pattern on the wafer with resist.
Typical EUVL projection objectives are co-axial optical systems. The object, image, and all the reflective elements are rotational symmetry. This feature is good for the correction of aberration and the manufacturing of the objective. Because of the folding and obstruction of the light path, the projection objective takes an off-axial field on object and image plane. Generally, there are several constraints for objective design except for known ones: 1. Achievable setting of a aperture diaphragm, which may set on one of the lenses 2˜5; 2. Enough working distance on both object and image side to ensure the space for mask and wafer setting; 3 non-obstruction design in the light path, all the used area on each element has enough space from the light path passed near it. 4. Suitable light path for the reflective mask, the incidental and emergent angle of ray beams on mask should be large enough to avoid obstruction with each other. 5. High resolution. 6. Extremly small distortions. 7. Teleconcentricity on the image plane.
Published document (M. F. Bal, Next-Generation Extreme Ultraviolet Lithographic Projection Systems[D], Delft: Technique University Delft, 2003) disclosed a method for EUVL projection objective design. The method makes exhaustive search on the first order parameters of the lenses of the objective with constraints of the magnification, conjugation relationship of the object and image, and then sifts the results of the exhaustive search by obstruction situation judgment, finally gets available structures of the objective, as “initial structures” for further optimization and design. The disadvantage of this method is that the calculation needed is too large for a common PC. It will take about a week to find an available structure.